An early compound engine that is comprised of a gas turbine unit and a piston unit is disclosed in the Johnston U.S. Pat. No. 3,498,053. In Johnston, the turbine unit is operated by the exhaust gases from a reciprocating two cycle, compression ignition, piston unit. The gas turbine unit of Johnston has a compressor component to supercharge the piston unit, and the turbine unit is also coupled to the output shaft of the piston unit. The Johnston piston unit has pairs of opposed cylinders with a scotch yoke connecting the pistons to a crankshaft.
A subsequent version of the compound engine is described in a publication entitled "A New Concept For Reduced Fuel Consumption In Internal Combustion Engines" presented at the 1971 Intersociety Energy Conversion Engineering Conference Proceedings, P38, by the Society of Automotive Engineers. This publication discloses a low pressure turbine mechanically coupled to the output shah of the piston unit and fluidly driven by the output of a high pressure turbine unit. The piston unit is a compression ignition two-stroke unit in which excess scavenge air flow in the valve-open portion of the stroke is used to internally cool the piston and cylinder. The exhaust valve is an annular valve located at the top of the cylinder and forms a portion of the wall of the combustion chamber. A bypass burner, that is, a thermal reactor, is placed in a parallel flow path between the high pressure compressor and high pressure turbine. To start the engine, a small starter motor spins the high pressure turbine, and combustion is initiated in the bypass burner. The bypass burner then becomes the combustion system to provide a driving gas for the high pressure turbine. The high pressure turbine first, mechanically drives the compressor to provide high pressure air to the piston unit, and second, provides a driving gas to the low pressure turbine which, in turn, provides cranking power to start the piston unit.
While the feasibility of many of the principles of the above engine configuration has been proven, there are several areas where components that are suitable for either only turbine engines, or only piston engines, are less suitable for compound engines and thus require further investigation and development. For example, the exhaust valves in typical compression ignition engines are generally of the poppet type and are actuated by an engine driven cam shaft which operates the valves through a rocker arm arrangement. The valves are held in a closed position by means of compression springs against which the rocker arm and cam operate to open the valves. As engine speed increases, the rate of occurrence of combustion cycles increases; and the speed with which the exhaust valves are opened and closed also increases. The speed with which the exhaust valves open is controlled by the engine driven cam. However, the speed with which the exhaust valves close can only be increased by increasing, or making larger, the spring constant of the valve return springs. While the stronger valve return springs facilitate operation of the engine at high speeds, these very strong valve compression springs have the disadvantage of requiring a very high torque to rotate the engine for starting, thereby making the engine more difficult to start, or otherwise turn over with an auxiliary starting motor or the low pressure turbine.
Therefore, to reduce the starting torque of the engine requires that the valve return springs be weaker; however, those weaker springs compromise the maximum speed of the engine because, as the engine speed increases, a speed will be reached at which the valve return springs will not be able to bring the valve to its fully closed position prior to combustion. Consequently, the exhaust valves will float in a constantly open position which has the disadvantage of governing or limiting the speed of the engine.
Conventional compression ignition engines address the high starting torque problem in several ways. First, a starter motor powered by a battery is used to start the engines, and an increased starting torque is accommodated by simply increasing the size of the starting motor and battery. In addition, an igniter can be used to provide ignition while the cold engine is being started. Often a bleeder valve is operable when the engine is being started to bleed pressure from the cylinder during the compression stroke. While effective, all of the above solutions have the disadvantage of adding complexity, weight and cost to the engine. More importantly, such solutions interfere, or are incompatible with, a turbine and piston unit working together.
Compound engines, and in particular the compound engines described above, have a piston unit with specially configured exhaust valves and combustion chambers. Further, the combination of the gas turbine unit with the piston unit presents difficult gas flow and energy transfer requirements. The valve actuations proposed by the prior art create additional problems in the manufacture and operation of such a compound engine. Accordingly, there remains a need for a solution to the valve actuation problems associated with compound engines of the type to which the present invention relates.